=over

=item dbmopen HASH,DBNAME,MASK
X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>

[This function has been largely superseded by the
L<C<tie>|/tie VARIABLE,CLASSNAME,LIST> function.]

This binds a L<dbm(3)>, L<ndbm(3)>, L<sdbm(3)>, L<gdbm(3)>, or Berkeley
DB file to a hash.  HASH is the name of the hash.  (Unlike normal
L<C<open>|/open FILEHANDLE,MODE,EXPR>, the first argument is I<not> a
filehandle, even though it looks like one).  DBNAME is the name of the
database (without the F<.dir> or F<.pag> extension if any).  If the
database does not exist, it is created with protection specified by MASK
(as modified by the L<C<umask>|/umask EXPR>).  To prevent creation of
the database if it doesn't exist, you may specify a MODE of 0, and the
function will return a false value if it can't find an existing
database.  If your system supports only the older DBM functions, you may
make only one L<C<dbmopen>|/dbmopen HASH,DBNAME,MASK> call in your
program.  In older versions of Perl, if your system had neither DBM nor
ndbm, calling L<C<dbmopen>|/dbmopen HASH,DBNAME,MASK> produced a fatal
error; it now falls back to L<sdbm(3)>.

If you don't have write access to the DBM file, you can only read hash
variables, not set them.  If you want to test whether you can write,
either use file tests or try setting a dummy hash entry inside an
L<C<eval>|/eval EXPR> to trap the error.

Note that functions such as L<C<keys>|/keys HASH> and
L<C<values>|/values HASH> may return huge lists when used on large DBM
files.  You may prefer to use the L<C<each>|/each HASH> function to
iterate over large DBM files.  Example:

    # print out history file offsets
    dbmopen(%HIST,'/usr/lib/news/history',0666);
    while (($key,$val) = each %HIST) {
        print $key, ' = ', unpack('L',$val), "\n";
    }
    dbmclose(%HIST);

See also L<AnyDBM_File> for a more general description of the pros and
cons of the various dbm approaches, as well as L<DB_File> for a particularly
rich implementation.

You can control which DBM library you use by loading that library
before you call L<C<dbmopen>|/dbmopen HASH,DBNAME,MASK>:

    use DB_File;
    dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
        or die "Can't open netscape history file: $!";

Portability issues: L<perlport/dbmopen>.

=back